Preparation And Reaction Study Of Supported Molybdenum Carbide Catalyst For Conversion Of Syngas Into Low Carbon Alcohols

Due to the increasing exhaustion of fossil fuels and the deterioration of atmospheric environment,it is urgent for human society to develop a novel sustainable"green energy"as a substitute.In particular,alcohol-based fuel is expected to be one of the green and environment-friendly fuels for vehicles in the future in terms of its several prestigious advantages,including low cost,rich in source,explosion-proof and quake-resistant,high octane number,and good environmental compatibility.On the other hand,high value-added chemical products can be obtained after its separation and refining,which can provide raw materials for some chemical production,with abundant after-processing routes and higher value-added derivatives.The energy composition of China is characterized by"more coal and less oil",thus the preparation of alcohol-based fuel from coal gasification syngas is of great significance to alleviate the energy crisis and open up the coal chemical industry chain.However,there exist many factors affecting the reaction activity and selectivity,among which the catalyst plays the most critical role.Molybdenum based catalysts show excellent catalytic performance due to the special noble-metal-like properties,so they are regarded as very promising catalysts for the synthesis of low carbon alcohols,but still many technical problems and bottlenecks exist in the preparation of high perfromance molybdenum carbide catalysts.In this paper,a series of supported molybdenum carbide catalysts with different morphologies and excellent catalytic performance were prepared by the solid-solid reaction method,with the solid organic carbon source with different morphologies as the carrier and ammonium molybdate as the molybdenum source.The effects of morphology,surface structure,active component and preparation process on the properties of molybdenum carbide catalyst were investigated by a variety of characterization methods.In addition,the structure-activity relationship among reaction conditions,additives and active components in the preparation of higher alcohol from syngas was revealed and elucidated.The major research contents and results are summarized as follows:（1）Preparation of lignin-based carbon material（MNT）with ordered porous structure by using sodium lignin sulfonate and cellulose based carbon（CEL）with fiber structure as raw materials;Preparation of graphite-like carbon nitride（g-C₃N₄）with lamellar structure from melamine.These as-prepared carbon materials were used as the carbon sources and the catalyst supports,and ammonium molybdate was used as the molybdenum source,and they were used to construct three supported molybdenum carbide catalysts namely,β-Mo₂C@CEL,β-Mo₂C@MNT andβ-Mo₂C@-C₃N₄,by a solid-solid reaction（carbon-thermal）method.Advanced analytical tools such as XRD,SEM,EDS,BET and TG-MS were used to characterize the three catalysts with different morphologies.SEM and BET analysis showed that the three supported catalysts still maintained the same morphology as the supports but obtained a better porous structure with increased surface area.The results of TG-MS and XRD showed that the natural doping elements in the support with three different morphologies had a direct effect on the carbonization temperature of the catalyst,and the carbonization temperature was in th order of MNT>CEL>g-C₃N₄.The relationship between the preparation of supported molybdenum carbide catalysts with different morphologies,pore sizes,specific surface areas and doping elements was investigated,and the preparation mechanism of catalysts was proposed.（2）A series of Ni-β-Mo₂C-K@MNT,Ni-β-Mo₂C-K@CEL and Ni-β-Mo₂C-K@-C₃N₄ catalysts were prepared by loading additional active components onto the support through incipient-wetness impregnation method,and the factors affecting the reaction performance of the catalysts were investigated with the catalyst preparation process.The phase structure,morphology and active component of the catalysts were analyzed by different characterization methods,with an emphasis on the discussion of the relationship between the support,additives,reaction conditions and the performance of the catalyst.By adjusting and balancing the two active sites in the catalyst,it was found that the catalytic performance of the catalysts was optimised.K promoter inhibited the selectivity of hydrocarbon compounds,thus enhancing the selectivity of alcohols while the addition of Ni additives is beneficial to the formation of hydrocarbon compounds.It should be noteworthy that the catalyst Ni_0.5M_40%K_0.2@g-C₃N₄ exhibited excellent catalytic activity at 360℃,8 MPa,and the space speed of 5000 h^-1(CO conversion was 55.58%,the selectivity of total alcohol was45.46%,the selectivity of C₂₊OH was 83.96%,and the yield was 0.33 g/g_cat/h).（3）Based on the discussions and research on the preparation and catalytic performance of the three catalysts,it was concluded that the supported molybdenum carbide catalyst with high dispersion,small particle size,high specific surface area and superior crystal structure could be prepared by utilizing carbon sources with a unique layer structure,superior porosity,specific surface area and the natural doping elements,as the supporst.The as-prepared catalysts showed excellent catalytic performance towards conversion of syngas to high-level alcohols,which could offer an important reference for the preparation of catalysts with high selectivity.